1. Field of the Invention
The present invention relates to a wireless communication system and method, and more particularly to a wireless communication system capable of performing an optimized routing and a method of measuring a magnitude of a network.
2. Description of the Related Art
With the development of the hardware technology and the increase in demand for portable terminals such as notebook computer, PDA (Personal Digital Assistant), etc., there exists a need for grafting the concept of wireless mobile communications on data communications based on the existing Internet protocol. A representative basic technology for this is an MP (Mobile IP) (hereinafter referred to as ‘mobile IP’).
Currently, a host, which uses a mobile IP in a wireless LAN (Local Area Network) environment, performs a handoff in two of OSI (Open System Interconnection) layers when the host moves from a present cell and moves to a new cell. A handoff performed in a MAC (Media Access Control) layer secures a reliable wireless link in the new cell, and a mobile IP handoff occurring in the IP layer provides a positional clarity of the host so that it is possible to continuously keep service during the movement of the host without changing the IP address.
A wireless LAN, which is a data communication system for providing mobility and scalability, facilitates its construction and management in comparison to the existing wire LAN, and currently provides a data transmission speed of approximately 11 Mbps. Also, the mobile host on the wireless LAN can receive an Internet service by connecting to the wire LAN at a high transmission speed at any location without a cable, i.e., wireless.
The standard of a physical layer and a data-link layer of the wireless LAN is described in the IEEE (Institute of Electrical and Electronics Engineers) 802.11. The wireless LAN is constructed by an ad-hoc network composed of a wireless terminal only or an infrastructure network that is connected to a wire LAN. A network in which nodes having mobility are autonomously constructed without any help of the existing infra-environment is referred to as a mobile ad-hoc network (MANET).
All nodes of the MANET are mobile; therefore, the structure of the network is dynamic and flexible. The MANET having such flexibility becomes a matter of great concern as a core network technology to deal with mobile terminal environments that are omnipresent, but it has several technical problems to be solved.
The MANET has a characteristic quite different from an existing infra-environment; therefore a separate routing design is required to support the MANET. A MANET working group in the IETF (Internet Engineering Task Force) is currently researching this design. Respective routing protocols discussed in the MANET working group are briefly classified into a proactive system and a reactive system.
The proactive system is an initially used routing algorithm for the ad-hoc network, and is also referred to as a table-driven system. The proactive system is provided by applying a Bellman-Ford system used in the Internet of a wire environment to the ad-hoc network. The proactive system continuously routes information of all mobile terminals in the ad-hoc network, and a respective mobile terminal periodically transfers the routing information to other mobile terminals, and when changing a routing path, the respective mobile terminal broadcasts its own routing information. The advantage of the proactive system is that a delay time for obtaining a route is shortened by continuously keeping the routing information of other mobile terminals in a routing table, and thus directly transmitting data packets using the information of the routing table without performing a separate algorithm for searching for the route if the data packets to be transmitted are produced. However, since the proactive system should continuously and periodically broadcasts the routing information while the network is maintained, the number of packets of the routing information increases according to a square of the number of nodes in the ad-hoc network in which the use of radio frequencies is limited, and this burdens the network.
The reactive system is referred to as an on-demand system. The reactive system does not always keep the entire route for all of the mobile terminals in the network as in the proactive system, but instead obtains a route only when the data transmission is required. Since the reactive system does not keep the routing information, and obtains the routing information only in case of need, it is not required to broadcast the changed routing information to the mobile terminals when the mobile terminal moves. However, because data is transmitted after performing a procedure to obtain the routing information, the time required for obtaining the routing information increases, and thus the reactive system is unsuitable to a real-time communication.
The proactive system and the reactive system commonly transmit a control message for searching for or informing a newest route information to the whole network. The transmitting route for such a control message forms a minimum spanning tree.
In such a route discovery process, a timeout value for the control message is a very important factor that determines the performance of the system. If a large timeout value is set in a small-sized ad-hoc network, it takes a lot of time to perform a new route discovery process if the route discovery fails. By contrast, if a small timeout value is set in a large-sized ad-hoc network, the transmission range of the control message is limited, and the route discovery process may fail to search for a node located in a far-off location. Accordingly, the timeout value, which corresponds to a waiting time after the control message is transmitted to the whole network, is typically set depending on the magnitude of the network.
However, the routing protocols on the ad-hoc network do not propose any mechanism for measuring the magnitude of the network. In most cases, a specified constant value is set and assumed as the magnitude of the network for each routing protocol. Such assumption may cause a severe problem in performance due to an inaccurate recognition of the magnitude of the network when the control message is transmitted for the route discovery.
A ring-search algorithm has been proposed as a method of reducing the number of control messages when searching for the route in the MANET. A node that uses the ring-search algorithm transmits a message to adjacent nodes, and whenever the transmission fails, the node increases the transmission range and re-transmits the message. This algorithm is useful because it can reduce the number of messages transmitted when a destination node is adjacent to the transmitting node. However, when the destination to be searched for exists further away from the transmitting node or does not exist on the network, the limit for increasing the transmission range cannot be determined only by the corresponding algorithm. Accordingly, in order to apply the corresponding algorithm to the network, the range of the network to which the node belongs must be known in advance.